TypeError: "this.getData(...).map is not a function"

[dwwe2017]

TypeError: "this.getData(...).map is not a function" value List.js:175 s react-dom.production.min.js:130 beginWork react-dom.production.min.js:133 a react-dom.production.min.js:161 l react-dom.production.min.js:161 i react-dom.production.min.js:162 O react-dom.production.min.js:169 _ react-dom.production.min.js:168 m react-dom.production.min.js:167 p react-dom.production.min.js:165 enqueueSetState react-dom.production.min.js:108 setState React value ListExample.js:15 react-dom.production.min.js:164:440 s react-dom.production.min.js:164 i react-dom.production.min.js:162 O react-dom.production.min.js:169 _ react-dom.production.min.js:168 m react-dom.production.min.js:167 p react-dom.production.min.js:165 enqueueSetState react-dom.production.min.js:108 setState React value ListExample.js:15

Everything was done as described in the example, also the method names of the Api controller were corrected regarding the callback.